In a conventional powered machine, a prime mover can operate at different speeds and produce different levels of power that is transferred to a transmission. In one instance, the prime mover can be an engine. In turn, the transmission can transfer torque to a driveline or final drive assembly, which can be directly mounted to the wheels or tracks of the powered machine. The transmission can include an internal pump that is rotatably driven by the prime mover, and based on the different speeds of the prime mover, the pump can produce different levels of fluid flow and pressure. In some instances, there is only one internal pump in the transmission that provides fluid flow to a main pressure circuit and lube circuit of the transmission.
A conventional hydraulic pump is often designed as a result of its desired functionality. In an engine-transmission application, for example, a conventional hydraulic pump may be designed for one of several reasons, namely, 1) to provide adequate fluid flow at a low engine idle speed (e.g., approximately 500 RPM), 2) to provide full regulated pressure to the main pressure circuit of the transmission at a specific engine speed (e.g., approximately 1000 RPM), and/or 3) to fill a transmission clutch within a desired time period (e.g., approximately 200 ms at 1200 RPM). Other design considerations may include margin of safety and leakage at a fluid temperature of about 120° C. In view of the different design considerations accounted for in a hydraulic pump, however, the pump still often tends to overproduce fluid flow at or above normal operating conditions and engine speeds.
Moreover, once the hydraulic pump is able to provide adequate fluid flow to the control and lube systems of the transmission, additional fluid flow produced by the pump is generally returned to transmission sump and is unusable. This excess fluid flow, however, directly contributes to hydraulic spin-loss inside the transmission. In effect, this reduces transmission productivity and performance.
One possible solution to the excess flow produced by the hydraulic pump is to incorporate a variable displacement pump into the transmission design. A variable displacement pump can increase or decrease volume inside the fluid cavity of the pump, thereby controlling the pump displacement and production of fluid flow. By controlling displacement, the pump can produce a more desirable amount of flow under steady-state conditions. When the transmission is in a certain range, for example, the hydraulic demand is usually fairly low and the volume of the oil cavity can be decreased, thereby resulting is reduced overall pump flow. Likewise, during a shift between ranges, the hydraulic demand increases for filling a clutch such that the volume of the oil cavity is increased and more flow is produced to meet demand.
Since the “decrease” pressure is based off of pressure in the main circuit, however, there is an inherit response time drawback. In other words, the demand to increase fluid flow (e.g., when filling a clutch) begins before the volume of the pump cavity increases (“decrease” pressure responds). Thus, regardless of what improvements are made to the pump and transmission system, the hydraulic demand rises before the pump can supply the desired flow, thereby resulting in an undesirable time delay to fill the clutch. This can impact fuel economy and shift quality.
A need therefore exists for electronically controlling the pump capacity of the transmission. By controlling pump capacity, it is also desirable to control fluid flow from the pump to minimize excess flow once the different fluid circuits of the transmission are satisfied, improve shift quality, and control fluid temperature of the transmission.